Sepek was voiced by Keith Sutherland. The final draft of the script describes the following: "Sepek is larger, probably a year or so older than the others." In the "cast" portion of the script, his name was initially spelled "Seped".

The novelization of "Yesteryear" (in Star Trek Log 1) refers to Sepek as the tallest as young Spock's three bullies and goes on to describe him, in relation to Spock, as "a persistent childhood tormentor until later years, when they grew to become great friends."